Recently, blue-violet semiconductor lasers have been practical. The blue-violet semiconductor laser contributes to practical application of a blue-ray disc (referred to as “BD” hereinafter). Although BDs is as big as compact discs (CDs) and DVDs, BDs may have capacities large enough to store information at a higher density than the CDs and DVDs.
A BD is an optical disc with a protective substrate which is approximately 0.1 mm in thickness. With a light source, which emits a blue-violet laser beam having a wavelength of approximately 400 nm, and an objective lens having a numerical aperture (NA) up to “0.85”, information may be recorded and/or reproduced in/from the BD.
Under such circumstances, it has been proposed to record or reproduce information with a compatible optical pickup device, which uses one or more objective lenses to focus different light beams in wavelength onto information recording surfaces of plural optical discs that have different protective substrates in thickness.
Very accurate positional adjustment to optical elements such as optical lenses for the information process (reproducing or recording information) by using BDs is required although there are attempts of size, weight and cost reduction for the aforementioned information processing technologies with light. In addition, there are needs for optical elements which allow easy installation to optical pickup devices. Furthermore, because the size reduction makes optical elements less visible and is likely to cause erroneous assembly of optical elements, there are requirements for technologies to prevent the erroneous assembly.
Patent Document 1 (JP 2011-108350 A) discloses a lens fixing device which has a lens holder and a mount to which the lens holder is mounted.
FIG. 21 is a schematic front view of the lens holder 900 disclosed in Patent Document 1. The conventional lens holder 900 is described with reference to FIG. 21.
The lens holder 900 includes a base 910, in which a tens region 901 is formed, a first projection 920, which projects from the base 910, and a second projection 930, which projects from the base 910 like the first projection 920. The lens holder 900 with the lens region 901 is integrally molded by resin molding techniques.
The first projection 920 includes a guide surface 921 inclined with respect to the y-axis, and a gluing surface 922 on the x-axis. The second projection 930 includes a guide surface 931 inclined with respect to the y-axis, and a gluing surface 932 on the x-axis. The extended surface of the guide surfaces 921, 931 intersects with the extended surface of the gluing surfaces 922, 932 at the optical axis OA of the lens region 901.
The mount (not shown) onto which the lens holder 900 is mounted includes a receiving surface with which the guide surfaces 921, 931 come into contact. When the guide surfaces 921, 931 come into contact with the receiving surface, the lens holder 900 is positioned in the X-Y plane. The lens holder 900 may determine a single rotational direction around the optical axis of the lens region 901. According to Patent Document 1, the shape of the lens holder 900 simplifies work for fixing the lens and causes little misalignment of the optical axis of the lens which is caused by a change in temperature of the lens.
Patent Document 2 (JP 2003-156601 A) discloses other technologies about positioning of a lens. It should be noted that small optical lenses disclosed in Patent Document 2 are not applicable to optical pickup devices.
FIGS. 22A and 22B are schematic front views of lenses 940, 950 disclosed in Patent Document 2. The conventional lenses 940, 950 are described with reference to FIGS. 22A and 22B.
The lens 940 shown in FIG. 22A includes a lens portion 941 and a rectangular flange 942 surrounding the lens portion 941. The flange 942 includes a left surface 943 on the left of the lens portion 941 and a right surface 944 on the right of the lens portion 941.
The lens 950 shown in FIG. 22B includes a lens portion 951 and a hexagonal flange 952 surrounding the lens portion 951. The flange 952 includes a left surface 953 on the left of the lens portion 951 and a right surface 954 on the right of the lens portion 951.
The lenses 940, 950 have the flanges 942, 952, respectively, which have polygonal outlines. The left surfaces 943, 953 and the right surfaces 944, 954 are flat and longer than diameters of the lens portions 941, 951. The geometric characteristics of the flanges 942, 952 makes the lenses 940, 950 easily mounted and positioned in holders such as lens barrels. The flat left surfaces 943, 953 and the flat right surfaces 944, 954 are used for preventing the lens portions 941, 951 from being rotated with respect to holders such as lens barrels when the lenses 940, 950 are mounted into the holders.
Patent Document 3 (JP 2003-121716 A) discloses other technologies about positioning of a lens.
FIG. 23A shows a schematic front view and a schematic side view of a lens 960 disclosed in Patent Document 3. FIG. 23B shows a schematic front view and a schematic side view of a lens 970 disclosed in Patent Document 3. The lenses 960, 970 are described with reference to FIGS. 23A and 23B.
The lens 960 shown in FIG. 23A includes a lens portion 961 and a flange 962 surrounding the lens portion 961. The flange 962 is formed by cutting a part of a circular flange material surrounding the lens portion 961. Therefore, the flange 962 includes an arch contour portion 963 and a straight contour portion 964. The outline of the flange 962 is generally D-shaped.
The lens 970 shown in FIG. 23B includes a lens portion 971 and a flange 972 surrounding the lens portion 971. The flange 972 is formed by cutting a part of a circular flange material surrounding the lens portion 971 in the thickness direction. Therefore, the flange 972 includes an arch contour portion 973 and a straight contour portion 974. The flange 972 partially has a D-shaped contour.
The shapes of the flanges 962, 972 shown in FIGS. 23A and 23B contribute to improvement of accuracy and degree of freedom about positioning of the lenses. In particular, because the flanges 962, 972 allow positioning in a rotational direction, geometry of the flanges 962, 972 is advantageous if positioning is required in a rotational direction.
Patent Document 4 (JP 2009-266264 A) discloses an optical pickup device. The optical pickup device of Patent Document 4 includes a lens holder, which holds a detection lens, and an optical base, which receives the lens holder. The lens holder has a main body for holding the lens, and a bulging portion which is shifted in one direction from the center of the detection lens. The bulging portion bulges from the main body in a direction orthogonal to a direction of the optical axis of the detection lens. An abutting portion is formed in the bulging portion to come into line contact with a receiving surface of the optical base. The receiving surface and the abutting portion may come into line contact with each other in a direction parallel to the optical axis. The technologies disclosed in Patent Document 4 may precisely adjust the position of the detection lens.
With respect to the lenses described in Patent Documents 1 and 4, when the lens is fabricated separately from the lens holder, a clearance has to be created in the lens holder so that the lens may be inserted into the holder. Generally, a lens is bonded to a lens holder. Therefore, the clearance makes the lens position unstable under a temperature change. In addition, processes are required to attach the lens to the lens holder.
If the lens is integrally resin-molded with the lens holder, these problems may be solved. However, according to the technologies disclosed in Patent Documents 1 and 4, the outer shape of the lens is so complicated and asymmetric in the vertical and horizontal directions that performance of the lens after molding degrades. Poor separation from a mold in the lens molding process may be another problem.
The shape of the lens disclosed in Patent Document 2 does not contribute to setting a single rotational direction of the lens around the optical axis. On the other hand, the shape of the lens described in Patent Document 3 enables to set a single rotational direction of the lens around the optical axis. However, the smaller the lens is, the less visible the cutout part is. Because of the small cutout part, even when an operator inserts the lens in a wrong direction, it is difficult for the operator to notice such erroneous operation.    Patent Document 1: JP 2011-108350 A    Patent Document 2: JP 2003-156601 A    Patent Document 3: JP 2003-121716 A    Patent Document 4: JP 2009-266264 A